My general way of handling out what I think I will need is (per baby)
newborn stage: 18-24 diaper changes a day.
infant and older stage: 12-18 diaper changes a day.

That means how many things they can pee on. So that could be prefolds or aios or pockets. If it's prefolds/flats/etc that need a cover, then I usually get 1 cover for every 3-4 diapers. BUT if you don't mind washing the covers every day/hand washing them after they've been used a few times, you can easily get by on 6-12 covers per baby regardless how many diapers you have.

I don't suggest trading out all of them for newborn alva's, unless you do customs and get extra snaps on them. They have a pretty big leg gap. I think 12 covers and 6 longies will work fine, that is 9 covers each. You can always wash the covers separately if they are not quite enough. If nothing else adding 4 more covers/pockets that can be used either way, or wool should be more than adequate.